How do I light an interview with one light?

One good light beats four bad ones. Here's the working setup.

Position: 45 degrees off the camera axis, 45 degrees up, on the side the subject faces. That's the classic key — it gives the face shape and puts a triangle of light on the far cheek (Rembrandt's whole career was this).

Soften it: the bigger the source relative to the face, the softer the shadows. Punch your light through a diffusion frame, a shower curtain, or bounce it off a white wall. Get the soft source CLOSE — softness comes from relative size, and close also gives you faster falloff (background goes moody by itself).

The free second light is negative fill: hang something black (hoodie on a C-stand counts) on the shadow side. Killing the bounce deepens the contrast and makes one light look deliberate instead of broke.

Separate the background by distance, not light: pull your subject 6+ feet off the wall. The key's falloff darkens the background naturally, and any practical lamp in the deep background gives you a free bokeh accent.

Expose for the bright side of the face, let the shadow side fall where it falls, and you've got a setup indistinguishable from a three-light interview at a glance.